The author of the mega-selling “Harry Potter” series has an agreement with Little, Brown in the United States and Britain to publish her first novels for grownups. The title, release date and details about the novel, long rumored, were not announced Thursday.

Rowling’s seventh and final Potter story came out in 2007. The series has sold some 450 million copies and been translated into 67 languages. It is the best-selling book series of all time, and the eight movies it spawned comprise the highest-grossing film franchise ever. With tie-in merchandise factored in, the Harry Potter franchise has been estimated to be worth more than $15 billion.

Hachette, owner of Little, Brown, did not divulge whether Rowling was paid an advance for the novel.

‘New territory’
Rowling's Potter books were published by Bloomsbury in Britain and Scholastic in the U.S. Rowling will now share the same publisher with Stephenie Meyer, whose "Twilight" series at least partially filled the gap opened by the conclusion of the Potter stories.

According to the release from Little, Brown, Rowling commented: “Although I've enjoyed writing it every bit as much, my next book will be very different to the Harry Potter series, which has been published so brilliantly by Bloomsbury and my other publishers around the world. The freedom to explore new territory is a gift that Harry's success has brought me, and with that new territory it seemed a logical progression to have a new publisher. I am delighted to have a second publishing home in Little, Brown, and a publishing team that will be a great partner in this new phase of my writing life.”

There have been rumors for years that Rowling was working on a crime novel set in the Scottish capital of Edinburgh. However, in 2007 crime author Ian Rankin told the U.K. Guardian that the report, supposedly based on his wife Miranda having spotted Rowling “scribbling away” in an Edinburgh cafe, was “a joke that got out of hand.”

But the lack of details about the forthcoming book hasn’t stopped Rowling’s legion of fans from exulting and speculating on Twitter under the hashtag RowlingReturns. “I cannot express my excitement,” tweeted one.

Another fan tweeted: “Do we know what it's about? No. When it comes out? No. Are we on cloud nine anyways? You know it.”

Rowling's novel will be available in both print and electronic formats. The author held out for years on allowing the Potter books to come out digitally, but announced in 2011 that Potter e-books would be sold through her own "Pottermore" website. The books were supposed to become available last fall, but have been delayed until sometime this year.